You won't need to play "full" chords, anyway. Most stuff in rock music are your basic power chords with a solo and/or guitar riff.
The problem I see with the pro mode are the solos. You can't do that RB style (randomly pressing buttons and struminng) if you're bad. You will fail in no time on that real thing. A Weezer/Foo song, no problem. But try playing some sick Metallica solo, you have like no chance at all and that'll be the case for 99% of the players that buy that thing.
But I hope there's a practice mode where it says you how to place your fingers and when to change a chord, etc. That would be really awesome and I'd maybe even buy that thing just to learn some more complex songs. But I somehow doubt that as it would be very time-consuming for HMX to program that for every song or even only the songs on disc.